In the processing of petroleum hydrocarbons and feedstocks such as crude oil and petroleum processing intermediates, and petrochemicals and petrochemical intermediates, e.g., gas, oils and reformer stocks, chlorinated hydrocarbons and olefin plant fluids such as deethanizer bottoms the hydrocarbons are commonly heated to temperatures of 100.degree. to 1000.degree. F. Similarly, such petroleum hydrocarbons are frequently employed as heating mediums on the "hot side" of heating and heat exchange systems such as vacuum tower bottoms and slurry systems. In both instances, the petroleum hydrocarbon liquids are subjected to elevated temperatures which produce a separate phase known as fouling deposits, within the petroleum hydrocarbon. In all cases, these deposits are undesirable by-products. In many processes, the deposits reduce the bore of conduits and vessels to impede process throughput, impair thermal transfer, and clog filter screens, valves and traps. In the case of heat exchange systems, the deposits form an insulating layer upon the available surfaces to restrict heat transfer and necessitate frequent shut-downs for cleaning. Moreover these deposits reduce throughput, which of course, results in a loss of capacity with a drastic effect in the yield of finished product. Accordingly, these deposits have caused considerable concern to the industry.
Organic foulants are usually higher molecular weight materials ranging in consistency from that of tar to rubber to "popcorn" to "coke." The exact composition of such foulants is difficult to identify.
One particularly troublesome type of organic fouling is caused by the formation of polymers that are insoluble in the hydrocarbon or petrochemical fluid being processed. The polymers are usually formed by reactions of unsaturated hydrocarbons, although any hydrocarbon can polymerize. Generally, olefins tend to polymerize more readily than aromatics, which in turn polymerize more readily than paraffins. Trace organic materials containing hetero atoms such as nitrogen, oxygen and sulfur also contribute to polymerization.
Polymers are formed by free radical chain reactions. These reactions, shown below, consist of two phases, an initiation phase and a propagation phase. In reaction 1, the chain initiation reaction, a free radical represented by R., is formed (the symbol R can be any hydrocarbon). These free radicals, which have an odd electron, act as chain carriers. During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see reaction 2c), forming the unwanted polymers which accumulate on heat transfer surfaces.
Chain reactions can be triggered in several ways. In reaction 1, heat starts the chain. Example: when a reactive molecule such as an olefin or a diolefin is heated, a free radical is produced. Another way a chain reaction starts is shown in reaction 3. Here metal ions initiate free radical formation. Accelerating polymerization by oxygen and metals can be seen by reviewing reactions 2 and 3.
1. Chain Initiation
R--H.fwdarw.R.sup.. +H PA1 a. R.sup.. +O.sub.2 .fwdarw.R--O--O.sup.. PA1 b. R--O--O.sup.. +R'-H.fwdarw.R'.sup.. +R--O--O--H PA1 c. R.sup.. +C.dbd.C.fwdarw.R--C--C.sup.. .fwdarw.polymer PA1 a. Me.sup.++ +RH.fwdarw.Me.sup.+ +R.sup.. +H.sup.+ PA1 b. Me.sup.++ +R--O--O .sup.. +H.sup.+ PA1 a. R.sup.. +R'.sup.. .fwdarw.R--R' PA1 b. R.sup.. +R--O--O.sup.. .fwdarw.R--O--O--R
2. Chain Propagation
3. Chain Initiation
4. Chain Termination
Research indicates that even very small amounts of oxygen can cause or accelerate polymerization. Accordingly, to inhibit this insidious fouling problem, it is highly desirable to provide a polyfunctional process antifoulant which can, among other functions, inhibit oxygen based polymerization initiation. This antioxidant function serves as a "chain-stopper" by forming inert molecules with the oxidized free radical hydrocarbons, in accordance with the following reaction: